As is known, with the advancement of technology and the spread of devices for reproducing graphic elements printed or positioned on banknote paper, the need to have security elements which can be inserted at least partially in banknote paper has been, and still is, increasing. One of the elements that is constantly being developed and researched is the security thread that is normally inserted at least partially in banknote paper.
Through the years, the security thread has undergone a constant but significant advancement, so much that it is still one of the least counterfeited elements, since its counterfeiting is difficult even for expert counterfeiters.
Among security threads, it is possible to identify for example metallic threads with negative text, i.e., threads provided by a plastic backing on which at least one layer of metal is arranged in which characters, lettering and the like are formed by total removal of the metal at the regions of the characters and/or lettering.
EP 319 157 discloses this technique.
A thread is also known, for example, in which in order to increase the security characteristics, at least one layer of ink with fluorescent properties with a solid background is added and thus covers both the regions without metal and the metallic regions.
There are other types of thread in which discontinuities of the metal are inserted between blocks of letters so that conductivity can be detected in assured and known lengths.
However, the marketing of so-called “transfer” metallized stripes has allowed all these types of thread to be counterfeited significantly, leading to the need to provide a partially demetallized thread, which therefore leaves in the characters a small amount of metal which can be detected by suitable laboratory equipment.
Therefore, in this last type of thread, the characters are only partially demetallized. Patent applications PCT EP 02/111177 and PCT EP 04/004767 relate to so-called partially demetallized threads of the type described above.
There are also types of thread which can be detected also by means of magnetic sensors. EP 516 790 discloses a thread in which detectable magnetic regions are inserted between the letters provided by performing a total demetallization, so that the thread can thus be detected both as conductive (due to the presence of metal deposited in continuous form) and by way of magnetic sensors (by way of the magnetic elements arranged above or below the layer of metal but never inside the letters).
Therefore, EP 516 790 discloses a security device in which the characters or letters are totally demetallized.
Generally, coded magnetic threads are currently provided by depositing magnetic areas on a fully metallized polyester backing; such areas are composed with a single type of magnetic ink and are separated by spaces in which regions without metallic material, designed to generate texts, are generated. It is evident that once the presence of magnetic areas has been discovered, their coercivity and residual magnetism can be identified easily and accordingly the magnetic material to be used to create a counterfeit or forgery is also easily identifiable; a code thus provided generates the same signals, and therefore the same code, both when it is detected longitudinally (along the axis of the thread) and when it is detected transversely (reading at right angles to the thread).
Security threads with magnetic regions provided with a single type of magnetic ink also have been the subject of counterfeiting, again with the transfer method. It is in fact possible to provide a stripe which has negative characters by using metallic transfers onto which magnetic elements in continuous and discontinuous form are transferred at a later time between the blocks of letters in order to create magnetic codes. To provide industrially a thread of the type cited above it is sufficient to have a rotary printing machine with multiple printing sections as disclosed also in EP 516 790, printing on a transparent material (generally polyester) graphic markings with removable inks, metallizing in vacuum with a solid background so as to cover the removable inks, removing the inks and consequently also the metal that covers them, thus leaving graphic markings which are identical to the ones printed with removable inks, and subsequently printing with magnetic inks continuous or discontinuous areas in the regions that are not demetallized and are therefore adjacent to the demetallized regions.
In this manner, the presence of magnetic elements is not visible with normal viewing instruments (the naked eye, optical magnifiers, et cetera), since they are always covered by metal. The magnetic elements are visible exclusively with suitable devices dedicated to detecting magnetism, such as for example magnetic scanners or lenses with liquid magnetic ink inserted in vacuum.
Among sensors for reading security elements integrated in security papers, banknotes and the like, sensors such as those disclosed for example in EP 0428779 are known.
This patent in fact discloses a method which is based on identifying areas, separated by spaces, whose magnetic characteristics have a different coercivity, such as to generate two sequences of signals, which are generated by a first orientation magnet, by a first reading head, by a second orientation magnet for rotating the magnetism and by a subsequent second reading head with a further reorientation magnet. This reading method therefore provides for a plurality of elements arranged in succession.
However, the main drawback of such sensors is that they do not have the ability to “read” areas which are sufficiently close to each other and are therefore subject to the same degree of imperfect flatness, as occurs for example with used banknotes, which no longer have their original flatness in all their areas but can be creased and therefore no longer flat. Examining one another distant areas does not allow to have signals which can be compared to each other, since they originate from areas which differ at least in terms of flatness.